Just a quick point here, you cannot connect an LED to 12v with no resistor connected it will blow immediately as the resistor is what limits the current that flows through it
If you are unsure and are buying LEDs online (eBay) always buy the ones that are prewired with the resistor already attached (normally hss a small bit of black sleeving in one leg coming out of the LED) these can be connected straight onto a power supply and as above if wanted an additional resistor can be added to dim them down
If you are unsure and are buying LEDs online (eBay) always buy the ones that are prewired with the resistor already attached (normally hss a small bit of black sleeving in one leg coming out of the LED) these can be connected straight onto a power supply and as above if wanted an additional resistor can be added to dim them down